Process for the electrolysis of alkali metal halide brines



U. TSAO Sept. 17, 1968 PROCESS FOR THE ELECTROLYSIS OF ALKALI METAL HALIDE BRINES Filed April 27, 1965 mm w i M :8 55m 3.32am rah/2 n v mm w 3 6 Eumtm wea Wm WM I0 2 o om 6 52332 %N 0593mm 3:55am (w mm kiful vn N NW 0 I .3 sufiw .5 52 A /w\ 5 k QN v\ Q\ 6 0 .5 sum INVENTOR Utah Tsuo BY Willa/0n ATTORNEYS United States Patent 3,402,113 PROCESS FOR THE ELECTROLYSIS 0F ALKALI METAL HALIDE BRINES Utah Tsao, Jersey City, N.J., assignor to The Lummus Company, New York, N.Y., a corporation of Delaware Filed Apr. 27, 1965, Ser. No. 451,263 4 Claims. (Cl. 204-98) This invention relates to a method for supplying alkali metal halide brines for use in industrial processes requiring chlorine and, more particularly, relates to a method of supplying alkali metal halide brines for use in electrolytic cells for the production of chlorine and alkalies. The invention involves a method for supplying alkali metal halide brines to electrolytic cell systems which include both diaphragm cells and mercury cells.

Chlorine, for the most part, is produced commercially by the electrolysis of an alkali metal chloride brine, such as a sodium chloride brine, with the corresponding alkali metal hydroxide also being produced as a product of the electrolysis. In general, two different types of electrolytic cells are used to effect this electrolysis, i.e., diaphragm cells and mercury cells. Although in both types of cells, sodium chloride brine is electrolyzed using a carbon or graphite anode and chlorine and sodium hydroxide are recovered as the ultimate products, at this point the similarity between the two types of cells ceases.

In the diaphragm cell, the cathode is generally of metal, such as iron, and is separated from the anode by a permeable diaphragm, generally of asbestos. Additionally, in the diaphragm cell, the sodium hydroxide is recovered at the cathode in admixture with sodium chloride and sodium sulfate, which mixture is referred to generally as the cell liquor. This catholyte or cell liquor normally contains about 11% sodium hydroxide and 14 to 15% sodium chloride together with substantial quantities of sodium sulfate, and the sodium chloride must be separated from the sodium hydroxide. This separation is effected by the evaporation of the cell liquor to a concentration of of about 50% sodium hydroxide, at which strength the sodium chloride content ranges from about 0.8 to 2.0% depending upon the temperature of the sodium hydroxide solution. The above percentages as well as those noted elsewhere herein are percentages by weight, as is customary in the electrolytic chlorine-alkali industry with reference to percentages of components of solutions containing caustic soda.

In contrast, in the mercury cell, there is no diaphragm and the cathode is a moving film of mercury which passes through the cell. The sodium produced by the electrolysis of the brine forms an amalgam with the mercury, from which amalgam sodium hydroxide is recovered in concentrations ranging up to 70% without the necessity for evaporation. In addition to these differences in the component parts of the diaphragm and mercury cells, as well as the difference in the form in which the product sodium hydroxide is initially recovered, these types of electrolytic cells also differ as to the purity of the alkali metal halide brine feed required by each.

In the diaphragm cell, substantially complete removal of the calcium and magnesium impurities in the brine feed is essential in order to prevent diaphragm blockage. Additionally, for eflicient operation of the cell, it is desirable to maintain the sulfate content of the brine below about 5.0 g./ 1. Moreover, in diaphragm cell operation, it is necessary to provide a means for purging the sulfates from the system. On the other hand, for mercury cells, calcium impurities in the brine feed are not considered to be critical but magnesium impurities are so deleterious that substantially complete removal of these latter impurities is necessary. Additionally, substantially complete, removal of the heavy metals, such as iron, nickel, vanadium,

3,402,113 Patented Sept. 17, 1968 chromium and molybdenum, as well as aluminum, is essential inasmuch as these metals cause a break-down of the amalgam, thereby tending to cause a hydrogen discharge which leads to a dangerous concentration of hydrogen in the chlorine. As with the diaphragm cells, a sulfate content of only about 10 g./l. can be tolerated so that purging of the sulfate is likewise necessary. Moreover, in the operation of a mercury cell, where the brine is to be treated to remove impurities, it is necessary to dechlorinate the brine which has passed through the cell before treatment, resaturation and recycling of the brine to the cell.

Generally speaking, the installation costs of a mercury cell are slightly greater than those for a diaphragm cell. However, the mercury cell has an advantage in that the sodium hydroxide produced has a very low impurity content and is, thus, suitable for use in making rayon without further purification. In contrast, the sodium hydroxide from a diaphragm cell, being produced in a mixture with at least about equal portions of sodium chloride, is recovered from the evaporator with as high as 1% sodium chloride contained therein. To be suitable for many uses, such as for making rayon, this sodium hydroxide must be further purified, which purification increases the cost of the diaphragm cell sodium hydroxide to at least that of the mercury cell.

Inasmuch as all consumers do not require sodium hydroxide of mercury cell quality nor are they willing to pay a premium price for such a product, it is apparent that there are definite advantages to be obtained from an operation combining both mercury cells and diaphragm cells. These advantages stem chiefly from the fact that by such an operation it is possible to supply sodium hydroxide of either mercury cell quality or diaphragm cell quality, depending upon which is desired, without the additional expense incurred in purifying diaphragm cell sodium hydroxide to obtain caustic of useable quality.

In the coordinated operation of both diaphragm cells and mercury cells, the solid salt recovered from the diaphragm cells, by the evaporation of the cell liquor or catholyte, is used to saturate the circulating brine of the mercury cells, which brine is depleted in sodium chloride concentration in each pass through the mercury cell. In this manner, the salt recovered from the diaphragm cell can be utilized and there is a ready supply of solid salt for use in the mercury cells. However, as advantageous as such an operation might at first appear, difliculties therein have been encountered. Inasmuch as a high level of sulfate impurities cannot be tolerated in either the feed for the diaphragm cells or the mercury cells, in the past, it has been necessary to provide separate brine purification systems for both types of cells. Barium compounds are often used to precipitate the sulfate fro-m the recycled brine. Additionally, because of the detrimental effect of the chlorine which remains in the recycle brine on cell operation, dechlorination of this brine prior to treatment and recycle to the mercury cell has also been essential. It will be appreciated that the operation of separate purification facilities for both the diaphragm cell and mercury cell brine feed is expensive and tends to eliminate any cost advantage obtained by utilizing the solid salt recovered from the diaphragm cell to resaturate the mercury cell brine.

As noted above, it is suitable to resaturate the depleted brine from the mercury cell, but since sodium sulfate will accumulate in the mercury cell brine recycling system, a certain amount of the brine must be purged from the system to maintain a suitably low concentration of sodium sulfate. The purge from the mercury cell also contains some sodium chlorate, and if the purge stream is passed directly back to the brine circulation system of the diaphragm cell, the sodium chlorate content of the purge U stream will end up in the sodium hydroxide solution removed from the diaphragm cell. This impurity detracts from the value of the sodium hydroxide.

It is therefore a general object of the present invention to provide an improved process for the treatment of brine in diaphragm and mercury cells for the production of chlorine and caustic.

Another object of the present invention is to provide an improved process for the electrolysis of brine in diaphragm and mercury cells wherein sodium chlorate does not build up in the diaphragm cell circuit.

Still another object of the invention is to provide an improved method of controlling sulfate content of recycled brines, which method involves neither expensive reagents nor loss of substantial quantities of brine.

Various other objects and advantages of the invention will become clear from the following description of an embodiment thereof, and the novel features will be particularly pointed out in connection with the appended claims.

In essence, the present invention is based on the use of the purge stream from the mercury cell, which is relatively low in sodium sulfate, to wash the salt cake from the diaphragm cell, after repulping of the latter. This has the eifect of reducing the quantity of sodium sulfate carried into the mercury cell brine circuit, as without this washing, sodium sulfate will adhere to the salt crystals in the brine. The purge stream added to the repulping operation picks up sodium sulfate from the salt cake and is eventually purged from the system. Thus, the sodium chlorate contained in the purge stream from the mercury cell will not end in the caustic produced from the diaphragm cell. This scheme allows for a large purge stream without increasing the overall purge from the plant. Of course, a large purge stream is necessary if low sulfate concentration is to be maintained and precipitation of sulfate with barium compounds is to be avoided.

To purge the sodium sulfate accumulation in the diaphragm cell brine system, the salt produced in the evaporator is repulped by a recycle brine from the separator following repulping, plus added water as required to regulate concentration. This recycle brine will build up to a high concentration of sodium sulfate, and a part of this stream is purged to sewer to remove sodium sulfate accumulated in the system. Since the purge stream is high in concentration of sodium sulfate, the loss of sodium chloride is minimized.

Understanding of the invention will be facilitated by referring to the accompanying drawing which is a simplified schematic flow sheet or flow diagram in accordance with the invention.

With reference to the drawing, the raw brine from any convenient source, is first purified at for the removal of calcium and magnesium impurities and the like by conventional procedures. After purification, the brine is passed in line 12 to a resaturator 14, where salt in line 16 from the separator is added, in the manner discussed more fully below, and pH is adjusted as required for proper operation of the diaphragm cell. The saturated brine is then passed in line 18 to the diaphragm cell 20, where it is electrolyzed in the conventional manner, chlorine gas being recovered at the anode and the cell liquor, a mixture of sodium chloride, sodium sulfate and sodium hydroxide, is recovered at the cathode.

The cell liquor is removed from diaphragm cell 20 via line 22 and is passed to an evaporator 24 where a 50% NaOH solution is withdrawn to market or other usage, and the resulting salt cake is passed into the recycle circuit. The salt passes first in line 25 to repulper 26 where water from line 28 and brine in line 30 are added, the latter solution picking up a substantial quantity of the contained sodium sulfate.

The resulting slurry is then passed via line 32 to a separator 34, where it is washed with recycle brine from the mercury cell in line 31. Additional Wate is added at this stage as required from line 28. In the separator, separate salt and brine fractions are produced, the sodium sulfate (from the diaphragm cell) and sodium chlorate (from the mercury cell) content being concentrated in the latter fraction as noted above. The brine fraction is passed into purge line 35 and removed from the system, except for the portion employed in the repulping operation taken off in line 30.

As noted above, a portion of the salt fraction, low in sodium sulfate and sodium chlorate, is withdrawn from separator 34 and passed to resaturator 14 for use in the diaphragm cell. A second (and much larger) portion of the salt fraction passes in line 38 to saturator 40, where it is mixed with a portion of the untreated mercury cell purge stream in line 42 to provide the raw feed for he mercury cell. The pH may be adjusted in saturator 40 as required.

The saturated brine from saturator 40 is passed in line 44 to a filter 46, where graphite and insolubles are removed via line 48. The filtered brine is then passed in line 50 to the mercury cell 52, operated in the conventional manner, chlorine gas being withdrawn at the anode, the NaOH-mercury amalgam being withdrawn in line 54, the mercury stripped at 56 and recycled to the cell 52, and 50% NaOH being sent to market or other usage. The mercury cell liquor, rich in unreacted salt and low in sodium sulfate but containing sodium chlorate, is recycled in line 36 to separator 34 and saturator 40 as described above.

The process of the invention may be illustrated by the following data which are typical of a plant producing 250 t.p.d. of chlorine and 281 t.p.d. of caustic from diaphram cells and 150 t.p.d. of chlorine and 169 t.p.d. of caustic from mercury cells.

The stream entering repulper 26 through line 25 has the following composition, all figures being in pounds/ hour:

Table I NaCl 40,771 N3-2S04 NaOH 20 H O 21,000

There is no sodium chlorate in this stream, but as can be seen it is very high in sodium sulfate.

The division of the salt fraction between lines 16 and 38 is of course not equal, the latter being the main source for the mercury cell while the former is merely a recycle to the diaphram cell. For the conditions noted above, however, this fraction is divided two-to-one as follows:

Table II Line 16 Line 38 NaCl 12, 077 24,155 N H2304 25 50 N aClOg. O. 3 0. 6 1, 343 2, 037

It will be noted that neither of these streams contain any caustic, and that the sulfate concentration is quite low. Prior to considering the purge stream composition, it is of interest to consider the composition in line 36, from the mercury cell:

is relatively low in sulfate (compared to the diaphragm ccll concentration thereof noted in Table I above), but contains an appreciable amount of chlorate which would, except for the arrangement of the present invention, concentrate in the diaphragm cell circuit.

Lastly the composition of the purge from the system (line 35) after the recycle in line 30 illustrates the efliciency of employing the mercury cell liquor for repulping and washing of the diaphragm cell salt:

From this composition, it is clear that the great majority of both the sulfate and chlorate are concentrated in the purge stream and with a high volume of purge the total brine loss to the system is only about 18%. Yet, the concentration and separation are carried out without the use of extraneous reagents. In particular, of the 20 pounds per hour of sodium chlorate in the purge stream from the mercury cell, only 0.3 pound per hour is passed in the salt to the diaphragm cell plant; if the purge were recycled to the raw brine purification or resaturation sections, all of the sodium chlorate would pass to the diaphragm cell.

It will be understood that various changes in the details, materials, steps and arrangements of parts, which have herein been described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

What is claimed is:

1. In a process for the coordinated operation of diaphragm and mercury cells for the electrolysis of alkali metal halide brines, wherein salt and impurity sulfate recovered from the evaporation of the diaphragm cell catholyte is used to supply said mercury cell, the improvements comprising:

repulping said salt with purge brine, whereby a portion of the sulfate contained in said salt is taken into solution;

washing said repulped salt with depleted brine from said mercury cell, whereby sulfate content of said salt is further reduced, chlorate in said depleted brine remaining in solution; separating said salt-depleted brine mixture into a purge brine fraction and first and second salt fractions, said brine fraction being relatively concentrated in sulfate and chlorate and said salt fractions being relatively depleted in said sulfate and chlorate;

purging said brine fraction from the system, except as required for use in said repulping; dissolving said second salt fraction in fresh brine for electrolysis within said diaphragm cell, and

dissolving said first salt fraction in depleted brine from said mercury cell for electrolysis within said mercury cell.

2. The process as claimed in claim 1, wherein said alkali metal halide is sodium chloride, said sulfate is sodium sulfate and said chlorate is sodium chlorate.

3. The process as claimed in claim 1, wherein said first salt fraction saturates said depleted brine, and said second salt fraction saturates said fresh brine.

4. The process as claimed in claim 3, and additionally comprising filtering solid matter from said saturated depleted brine prior to passing same to said mercury cell.

References Cited UNITED STATES PATENTS 3,051,637 8/ 1962 Judice et al 20498 3,052,612 9/1962 Henegar et al 204-128 3,129,152 4/ 1964 Teske et al. 204128 FOREIGN PATENTS 528,331 7/ 1956 Canada.

JOHN H. MACK, Prim'my Examiner.

D. R. JORDAN, Assistant Examiner. 

1. IN A PROCESS FOR THE COORDINATED OPERATION OF DIAPHRAGM AND MERCURY CELLS FOR THE ELECTROLYSIS OF ALKALI METAL HALIDE BRINES, WHEREIN SALT AND IMPURITY SULFATE RECOVERED FROM THE EVAPORTION OF THE DIAPHRAGM CELL CATHOLYTE IS USED TO SUPPLY SAID MERCORY CELL, THE IMPROVEMENTS COMPRISING: REPULPING SAID SALT WITH PURGE BRINE, WHEREBY A PORTION OF THE SULFATE CONTAINED IN SAID SALT IS TAKEN INTO SOLUTION; WASHING SAID REPULPED SALT WITH DEPLETED BRINE FROM SAID MERCURY CELL, WHEREBY SULFATE CONTENT OF SAID SALT IS FURTHER REDUCED, CHLORATE IN SAID DEPLETED BRINE REMAINING IN SOLUTION; SEPARATING SAID SALT-DEPLETED BRINE MIXTURE INTO A PURGE BRINE FRACTION AND FIRST AND SECOND SALT FRACTIONS, SAID BRINE FRACTION BEING RELATIVELY CONCENTRATED IN SULFATE AND CHLORATE AND SAID SALT FRACTIONS BEING RELATIVELY DEPLETED IN SAID SULFATE AND CHLORATE; PURGING SAID BRINE FRACTION FROM THE SYSTEM, EXCEPT AS REQUIRED FOR USE IN SAID REPULPING; DISSOLVING SAID SECOND SALT FRACTION IN FRESH BRINE FOR ELECTROLYSIS WITHIN SAID DIAPHRAGM CELL, AND DISSOLVING SAID FIRST SALT FRACTION IN DEPLETED BRINE FROM SAID MERCURY CELL FOR ELECTROLYSIS WITHIN SAID MERCURY CELL. 